The major commercial route to high strength, high modulus carbon fiber is based on polyacrylonitrile precursors. Such fibers, which are useful as reinforcing elements, are generally prepared by heating the polyacrylonitrile fiber in an oxidizing atmosphere at 200.degree. to 400.degree. C. so as to form a cyclized structure in the fiber and then carbonizing the oxidatively cyclized structure at a higher temperature, generally above 800.degree. C. Increase in density is considered a good qualitative measure of cyclization (see Density Changes in Acrylic Fibers by Thermal Stabilization, Takaku et al, Sen i Gakkaishi, 38 (9), 82-8 (1982) and Carbon Work at the Royal Aircraft Establishment, W. Watt, Carbon 1972, 10, 121-143). The oxidative cyclization step is highly exothermic and releases .gtoreq.400 Joules/g of heat rapidly. If not controlled, this leads to deorientation and/or melting of the polyacrylonitrile fiber and results in low tensile properties in both stabilized and carbonized fiber. Improvements in control of this heat flux have been described in U.S. Pat. No. 4,336,022, wherein it is accomplished by use of ammonium sulfonate comonomers. Further improvements in control of heat evolution on oxidation are desirable and result from the present invention.